Ashworth began working for the Labour Party as a Political Research Officer in 2001, and was the Economics and Welfare Policy Officer from 2002-04.[6] In 2003, he was seconded to the Scottish Labour Party to work on the Scottish Parliament election campaign, where he worked closely with then-Chancellor of the Exchequer, Gordon Brown. Following the election, Scottish Labour remained the single-largest party at Holyrood and were re-elected, but the coalition with the Scottish Lib Dems remained in place.

When Gordon Brown became Prime Minister in June 2007, Ashworth was appointed deputy Political Secretary with the role of linking the Government to the trade unions.[5] There was speculation later that year that Ashworth might be selected to replace John Prescott as the official Labour candidate for Kingston upon Hull East,[9] although it came to nothing. Ashworth spent most of the Crewe and Nantwich by-election campaign in the constituency.[10]

After the Labour Party were defeated at the 2010 general election, Ashworth became Political Secretary to the acting party leader Harriet Harman. He did not publicly support any candidate in the subsequent leadership election because of his role working for Harriet Harman but he was described as a "key member" of Ed Miliband's team on the day after Miliband won the Labour leadership election.[11] When Miliband was elected as Leader of the Labour Party, he asked Ashworth to join his office as Head of Party Relations.[2]

With a general election imminent, Ashworth was identified as someone who the Labour Party leadership wished to find a seat for. He was linked with a possible candidature in Mansfield should the sitting MP Alan Meale decide to stand down, but Meale decided to seek re-election despite widespread speculation he was to retire from Parliament. Ashworth was then identified as a potential candidate for Nottingham East when the sitting MP John Heppell retired,[12] but the selection went to former MP Chris Leslie when the Labour National Executive Committee chose to impose Leslie at the last minute.[13]

Following his election as Labour Party leader, Jeremy Corbyn appointed Ashworth to the Shadow Cabinet role of Shadow Minister without Portfolio.[20] Ashworth was appointed Shadow Secretary of State for Health in October 2016. He has gone on record to say that a Labour government would not repeal the controversial Health and Social Care Act 2012 despite Labour's 2017 general election manifesto commitment to do so.[21]

In December 2015, Ashworth voted against the resolution to authorise RAF bombing of Syria.[22]

Ashworth is critical of problems in the NHS. Ashworth stated, “The 4.3 million patients on waiting lists and the nearly 27,000 patients who waited over 62 days for cancer treatment last year will feel sorely let down that reducing waiting lists and stamping out rationing isn’t the first priority of the new health secretary. Investment in technology is welcome but years of Tory austerity has seen hospitals build up a £5,000,000,000 repair backlog, resulting in clinicians nationwide using hundreds of pieces of equipment that are years out of date, as recently revealed by Labour. And commitments to prevention will ring hollow without reversing the substantial cuts to public health budgets, which are set to reach £800,000,000 by 2020/21."[23][24]

Ashworth is also concerned about closure of maternity units, Ashworth stated, “Expectant mothers deserve reassurance that the local maternity unit will be there for them when needed. It is a disgrace that almost half of maternity units in England had to close to new mothers at some point in 2017. The uncertainty for so many women just when they need the NHS most is unthinkable. Under this government, maternity units are understaffed and under pressure. Labour is committed to making child health an absolute priority with our ambition of the healthiest children in the world. That means giving every child the best start in life, including proper investment in maternity services.”[25]

Ashworth fears cuts to public health services are causing cancellations of children's operations. Ashworth stated, “Behind each of these cancellations is a sick child and their loved ones facing unnecessary distress for entirely avoidable reasons. These cancelled operations risk harming children further in the long term and it’s shameful that cancellations have reached this level [18647 children's operations cancelled in 2017]. The fact that thousands of children’s operations – including for broken bones, removing rotten teeth, eye surgery and even breast cancer – have had to be cancelled on this scale reveals yet again an NHS pushed to the brink by the Tories.”[26]

Ashworth expressed concern over staff shortages in the NHS, projected to increase during the coming decade. Ashworth maintains this is due to government mismanagement and financial restrictions on the NHS since 2010.[27]

Ashworth maintains the NHS needs extra support from the government to prevent a crisis in the winter 2018/2019. Ashworth stated, “There was no extra cash for acute trusts in the recent budget for winter and we know many hospitals are facing intense pressures following this year’s ‘summer crisis’. It would be unforgivable if patients suffer another winter like last year. [winter 2017/2018 when many patients spent hours in ambulances or on hospital trolleys because there was no room in wards] Ministers urgently need to reassure us they have a plan and will make proper funds available.”[28]

In December 2018 Ashworth was concerned about winter hospital provision and feared the government is unable to deal with the NHS due to pressure over Brexit. Ashworth stated, “This is a government dismally paralysed by the ongoing Tory civil war over the Brexit shambles. It will be an utterly embarrassing failure of leadership if the health secretary can’t get an NHS plan published because of the ongoing squabbling. A&Es are already overcrowded at unsafe levels this winter, waiting lists are getting longer and staff shortages across the NHS becoming more chronic. We need urgent action now. If the government can’t even get its proposals for the NHS agreed internally, then it should stand aside for a Labour government that will put patients first over this debilitating Tory civil war.”[29]

Ashworth became engaged to Emilie Oldknow, the East Midlands Regional Director for the Labour Party, in 2008. She was the official Labour candidate for Sherwood at the 2010 general election, but was not elected.[30] Former Prime MinisterGordon Brown and his wife Sarah attended the couple's wedding on 3 July 2010 in Derbyshire.[31] They have a daughter, Gracie, born in May 2011 shortly after his by-election victory,[32] and a second daughter.